The present invention relates generally to lithium ion, non-aqueous secondary electrochemical cells, and to batteries formed of such cells and, more particularly, to the use of carbon-carbon composite material as the active material for the negative electrode of such cells and batteries, to improve cycle life and self discharge characteristics of such cells and batteries.
Since its introduction and commercialization in 1991, rechargeable (or secondary) lithium-ion battery systems have received considerable interest not only to the battery community but also to the electronic industries. In lithium-ion batteries, carbon or graphite is used as an anode, a lithiated transition metal intercalation compound is used as a cathode and LiPF6 is used as an electrolyte in carbonate-based organic solvents. For example, the reactions at the electrodes and overall cell reaction of an oxide-containing lithium intercalation compound are as follows: 
where LiMO2 represents the lithiated metal oxide intercalation compound.
The electrochemical process is the uptake of lithium ions at the anode during charge and their release during discharge, rather than lithium plating and stripping as occurs in metallic lithium rechargeable battery systems. As metallic lithium is not present in the cell, lithium-ion cells provide enhanced safety and a longer cycle life than the cells containing metallic lithium. Because of their advantageous characteristics, lithium-ion batteries are widely used for consumer electronics applications such as cellular phones, laptop computers, camcorders, and personal digital assistant.
At present, special type of hard carbon or graphite is used as active anode material in commercial lithium-ion batteries. Polyvinyledene fluoride (PVDF) is used as a binder to improve the mechanical integrity of the electrode. Copper is universally used as the substrate for anode. Hard carbon or graphite material is mixed with PVDF in an organic solvent (N-methyl pyrolidone or dimethyl formamide) and the mixture is coated on the copper substrate to produce the anode.
Recently, there are a number of patents issued (e. g., Fong et al., U.S. Pat. No. 5,028,500; Bito et al., U.S. Pat. No. 5,580,538; Yoshimoto et al., U.S. Pat. No. 5,158,578; Nagamine et al., U.S. Pat. No. 5,667,914; Xue et al., U.S. Pat. No. 5,698,340; Yamada et al., U.S. Pat. No. 5,834,138; Nagamine et al., U.S. Pat. No. 5,932,373; Yamada et al., U.S. Pat. No. 5,972,536; and Yamada et al., U.S. Pat. No. 6,042,969) on the development of carbon materials as anode of secondary non-aqueous electrochemical cells.
During charge-discharge process, due to intercalation and de-intercalation of lithium-ions, a significant expansion and contraction of anodes occurs that can loosen the mechanical integrity and thereby impedance of the electrodes. This increase in impedance of the anode causes capacity fade of lithium-ion batteries during cycling. The present state-of-the-art lithium-ion battery delivers approximately 500 cycles at 100% depth of discharge with 80% capacity retention. There are many applications (e.g., aerospace and transportation) that demand higher cycle life.
Another disadvantage of the state-of-the-art lithium-ion battery is its relatively high self-discharge. The present lithium-ion battery loses 7% to 12% capacity per month at ambient temperature. The loss is even higher at higher temperatures.
Accordingly, it is the primary objective of the present invention to improve the cycle life of lithium-ion electrochemical cells and battery systems using such cells.
Another objective of the present invention is to improve the self-discharge characteristics of lithium-ion electrochemical cells and battery systems using such cells.
Still another objective of the present invention is to provide a novel and improved rechargeable lithium-ion cell and battery system which utilizes carbon-carbon composite material as anode.
Briefly stated, this invention provides a secondary electrochemical cell comprising a body of aprotic, non-aqueous electrolyte, first and second electrodes in effective contact with said electrolyte, the first electrode comprising an active material such as lithiated intercalation compound and the second electrode comprising carbon-carbon composite material. In accordance with this invention, commercially available carbon-carbon composite material of high electronic conductivity which also provides high lithium-ion intercalation capacity is chosen for the negative electrode, i.e., anode of the electrochemical cell. The carbon-carbon composite anode itself is also used as current collector. The mechanical strength of the carbon-carbon composite anode is superior to the carbon or graphite anode used in commercial lithium-ion batteries. The composite can accept repeated expansion and contraction as a result of intercalation and de-intercalation of lithium-ions during charge-discharge process with a little or no change in mechanical integrity. The impedance of the anode, therefore, remains almost the same. The cycling behavior of the lithium-ion cells made with the carbon-carbon composite shows significant improvement.
Carbon-carbon composite electrode consists of a single phase, does not contain any binder and there is no metal carbon interface. The self-discharge behavior of an electrochemical cell made with the carbon-carbon composite material as anode is, therefore, improved.